The present invention relates to a process for producing a coating film, a coating film produced by said process and a solid polymer electrolyte fuel cell having said coating film as an electrolyte membrane. Particularly, it relates to a process for producing a coating film which makes it possible to make strength properties of the film containing a reinforcing material comprising short fibrous fillers isotropic, a coating film produced by said process and a process for producing an electrolyte membrane for solid polymer electrolyte fuel cells by said coating process.
Fuel cells are expected to be widely used in the future since their power generation efficiency is high, and their load to the environment is light. Particularly solid polymer fuel cells are expected to be widely spread for movable bodies such as automobiles, or as a diversed power generation system or a cogeneration system for home use, since their power density is high and their operating temperature is low, whereby downsizing and cost cutting are easy as compared with other fuel cells.
In general, as illustrated in the sectional view of FIG. 7, a membrane-electrode assembly 101 for solid polymer electrolyte fuel cells comprises a polymer electrolyte membrane 103 comprising an ion exchange resin, catalyst layers 105a and 105b bonded to both sides of the polymer electrolyte membrane 103, and e.g. carbon paper or carbon cloth as gas diffusion layers 107a and 107b disposed outside the catalyst layers.
Outside the gas diffusion layers 107a and 107b, an electrically conductive separator 109 is disposed. On the separator 109, gas flow paths 111a and 111b, which face the gas diffusion layers 107a and 107b, are formed. A fuel gas and an oxidant gas are made to pass through the gas flow paths, and at the same time, electricity is transmitted from the gas diffusion layers 107a and 107b to the outside, and electric energy is taken out.
As described above, the membrane-electrode assembly 101 is formed by bonding the electrode catalyst layers 105a and 105b containing a noble metal on both sides of the polymer electrolyte membrane 103. The electrode catalyst layers 105a and 105b are formed by a method of directly coating the polymer electrolyte membrane 103 with an ink for formation of an electrode catalyst layer, containing a catalyst-supported carbon and a dispersion of an ion exchange resin (such as a dispersion of a perfluorocarbon polymer having sulfonic acid groups) as the main solid contents or a method wherein catalyst layers 105a and 105b preliminarily formed in the form of a sheet are bonded to the polymer electrolyte membrane 103 by means of e.g. hot pressing.
In addition, a method of coating each of the coating layers 105a and 105b formed into a sheet with an ion exchange resin dispersion by cast film forming, laminating and bonding and the catalyst layers 105a and 105b with the coating films faced inside, may, for example, be mentioned.
In order to improve performances of the fuel cell, it is considered to decrease the electric resistance by making the polymer electrolyte membrane 103 thin. In a case where a polymer electrolyte membrane 103 in the form of a thin film is formed by cast film forming by using an ion exchange resin comprising a fluorine-containing polymer having sulfonic acid groups, a method of mixing short fibrous fillers with the ion exchange resin with a purpose of compensating for the decrease in mechanical strength, is considered.
In a conventional cast film forming, as illustrated in a perspective view of FIG. 8, a die 121 for discharging a coating liquid is equipped with a linear opening 123 in the form of a slit downward as an exit. A substrate 125 for coating disposed to face the linear opening 123 is relatively movable in at least one direction. For example, the substrate 125 for coating is movable in the longitudinal direction X for feeding operation.
The coating liquid discharged from the linear opening 123 of the die 121 is coated on the coating substrate 125 by a coating operation which relatively moves the die 121 in the longitudinal direction of the coating substrate 125. By this cast film forming, a coating film consisting of a single coating layer 127 or a coating film consisting of a plurality of coating layers wherein a second coating layer 129 is further formed on the coating layer 127 formed in advance.
However, in a case where an ion exchange resin containing short fibrous fillers is coated by the cast film forming, the short fibrous fillers are likely to be orientated in one direction at the exit of the die, and anisotropy is generated such that the strength is different as between in the MD direction (the direction in which a film is formed, the direction of the arrow X in FIG. 8) and in the TD direction (the direction perpendicular to the MD direction). Namely, the reinforcing effect by the short fibrous fillers is restricted to one direction, and no adequate strength can be obtained depending upon the direction in the film plane.
Under these circumstances, it is an object of the present invention to provide a process for producing a coating film, which makes it possible to make strength properties of a film containing a reinforcing material comprising short fibrous fillers isotropic, a coating film produced by said process, and a process for producing an electrolyte membrane for solid polymer electrolyte fuel cells by said coating process.
The present invention provides a process for producing a coating film consisting of a single coating layer or a plurality of coating layers laminated, which comprises a coating operation of relatively moving at least one of a die for discharging a coating liquid containing short fibrous fillers from a linear opening with a predetermined length and a substrate for coating, on which the coating liquid discharged from the die is coated, to form a coating layer on the substrate for coating, wherein the direction in the coating operation includes at least two different angle directions with regard to the single coating layer or the plurality of coating layers of the coating film.
The coating film is formed by a coating operation in at least two angle directions. Here, the short fibrous fillers in the coating liquid are aligned mainly along the direction of the coating operation. Accordingly, the short fibrous fillers in the single or the plurality of coating layers are aligned in at least two different directions depending upon the coating operation. Namely, with regard to a cross section in an optional direction, the short fibrous fillers are present so that the short fibrous fillers in at least one direction cross the cross section.
Therefore, the coating film according to the production process of the present invention is free from such a drawback that mechanical properties such as a tensile modulus of elasticity and a tear strength are high only in one direction, and anisotropy in strength properties of a coating film such that the film is made to have a high strength only in one direction by short fibrous fillers, as in a conventional cast film obtained by moving the substrate for coating relatively to the die only in one direction along one line, can be decreased.
Further, in the present invention, the coating operation is preferably a composite operation comprising a feeding operation in one direction along one line and a reciprocating operation in a direction at right angles to the line of the feeding operation.
By the reciprocating operation under the process of the feeding operation, operations in at least different angle directions are continuously carried out, whereby a continuous film can be formed with coating operations in at least two directions. Further, the coating film can be formed by a simple construction comprising drive mechanism for the feeding operation and drive mechanism for the reciprocating operation.
Further, in the present invention, it is preferred that the direction in the feeding operation is a direction at right angles to the longitudinal direction of the linear opening of the die, and the direction in the reciprocating operation is the longitudinal direction of the linear opening of the die.
By the reciprocating operation in the die width direction with the feeding operation in a direction at right angles to the die width direction i.e. the die front direction, a coating film in a single layer can be formed. Further, as the directions in the feeding operation and in the reciprocating operation are at right angles, the film thickness can easily be controlled by adjusting the operation speed in both directions or the flow amount of the coating liquid.
Further, in the present invention, it is preferred that the direction in the feeding operation is the longitudinal direction of the linear opening of the die, and the direction in the reciprocating operation is a direction at right angles to the longitudinal direction of the linear opening of the die.
By the feeding operation in the die width direction with the reciprocating operation in the direction at right angles to the longitudinal direction (width direction) of the linear opening, i.e. the die front direction, a coating film wherein part of the layer is superposed on the other in every reciprocating operation, is formed. The proportion of superposition in the coating film can be changed depending upon the feeding speed of the die to the side.
For example, by determining the feeding speed of the substrate for coating so that it moves a distance of the length of the die in one cycle of the reciprocating operation of the die, superposed portions wherein two layers are superposed one on the other are continuously formed adjacent to each other. As a result, a laminated film consisting of two coating layers at every portion can be obtained.
Further, in the present invention, it is preferred that the coating film has at least two coating layers, in any two coating layers, each of the directions in the coating operations for forming the upper side coating layer and the lower side coating layer is at right angles to the longitudinal direction of the linear opening of the die, and after formation of the lower side coating layer, the direction of at least one of the die and the substrate for coating is changed to form the upper side coating layer so that the directions of the coating operations for the lower side coating layer and the upper side coating layer are at right angles to each other.
The coating film obtained by the above process is a laminate comprising at least two coating layers, and containing two layers of which the directions of casting are at right angles, whereby isotropy in strength is improved. Further, coating of this coating film can easily be carried out by superposing a plurality of coating layers, without a complicated coating operation.
Further, in the present invention, the short fibrous fillers preferably comprise a fibrillated fluorocarbon polymer.
In the present invention, the short fibrous fillers are meant for fibers having an average length of less than 3 mm. In a case where a polymer is contained in the coating liquid as a matrix to form a film, preferred is one which may function as a reinforcing material in the obtained polymer film. The diameter of the short fibers is usually at a level of from 0.01 to 10 xcexcm, and preferred are ones having a diameter at a level of from 0.05 to 5 xcexcm and a length at a level of from 1 xcexcm to 1 mm.
Specifically, glass fibers, silica fibers, quartz fibers, polymer fibers, metal fibers, ceramic fibers or carbon fibers may, for example, be mentioned. The polymer constituting the polymer fibers may, for example, be an aromatic polyamide, polybenzimidazole, polyparaphenylene benzobisoxasol, polypropylene, polytetrafluoroethylene (hereinafter referred to as PTFE), a tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer or a fluorocarbon polymer such as polyvinylidene fluoride.
Particularly when the coating film obtained by the present invention is applied to e.g. fuel cells, preferred are fibers comprising a fluorocarbon polymer in view of improvement in elastic modulus while relatively securing durability and toughness.
Among them, fibrils comprising a fluorocarbon polymer are preferred. Specifically, preferred are fibrils of PTFE. The fibrils of PTFE may be obtained by imparting shear force to a powder of PTFE, and they can increase the tensile modulus of elasticity and tear strength even in a small content.
Further, the coating film obtained by the above process is preferably solidified or cured by reaction.
The coating film thus obtained is excellent in tear strength properties not only in one direction, but the anisotropy in strength of the coating film by the short fibrous fillers is decreased. Accordingly, a thin film with a small anisotropy can be formed by cast film forming.
The present invention further provides a process for producing an electrolyte membrane for solid polymer electrolyte fuel cells comprising a reinforced ion exchange membrane, wherein the ion exchange membrane is produced by the above-described process for producing a coating film, and the coating liquid contains short fibrous fillers and an ion exchange resin.
The present invention still further provides a process for producing a solid polymer electrolyte fuel cell comprising a polymer electrolyte membrane and an anode and a cathode facing each other with the polymer electrolyte membrane interposed therebetween, wherein the polymer electrolyte membrane is produced by the above-described process.
By blending short fibrous fillers comprising e.g. a fibrillated fluorocarbon polymer into the coating liquid containing an ion exchange resin, a thin film with small anisotropy can be formed by cast film forming using the coating liquid, and the obtained coating film has a low electric resistance. By disposing this film as a polymer electrolyte membrane, a solid polymer electrolyte fuel cell having high performances can be constituted.